Oxidative damage to tissues has become a recurring theme as a mechanism for the induction of a variety of medical conditions including myocardial ischemia, cancer, and aging. Myoglobin, a heme protein that is ubiquitous in aerobic muscle tissues, has peroxidase activity that can result in oxidative damage to a variety of biological molecules, including itself and membrane lipids. Since the 1950's, metmyoglobin (metMb) has been known to react with hydrogen peroxide, resulting in the one-electron oxidation of ferric heme to form ferryl heme. The fate of the second oxidizing equivalent available from the reduction of hydrogen peroxide to water has been in question since the reaction was first reported. Recent results support the formation of a protein-centered radical, but which amino acids are involved is still under debate. Before the identity of the latter amino acid residue was known, we demonstrated with direct ESR that a peroxyl radical does form, but that this peroxyl radical does not form a detectable radical adduct. In addition, this peroxyl radical is formed by the reaction of molecular oxygen with the unknown amino acid radical. Both the peroxyl radical and its precursor radical oxidize cellular constituents such as glutathione, but only the peroxyl radical reacted with PUFA (initiating lipid peroxidation) and styrene (in a reaction known to form styrene expoxide). The peroxyl radical was unambiguously characterized by its g tensor and 17O hyperfine coupling from both oxygens originating from 17O-labelled molecular oxygen. In the last published investigation, we utilized the ESR spin-trapping technique with the nitroso-based spin traps to identify the amino acid residue radical which is the precursor to the peroxyl radical. The spin trapping results demonstrate that the globin radical that has been spin trapped is centered on a tryptophan residue. Since free tryptophan radical reacts with oxygen, it is likely that a tryptophan radical in a protein would also do so to form a peroxyl radical.